Carboxylated rubbers (rubbers containing carboxyl groups in their polymer chain) are useful for many purposes. Carboxylic nitrile rubber (XNBR) is a terpolymer of butadiene, acrylonitrile, and methacrylic acid. This carboxyl modification of nitrile rubber (NBR) produces a material that has outstanding abrasion resistance. Metal oxide vulcanizates of carboxylic elastomers also have unusually high tensile strengths, superior ozone resistance, and elevated modulus values. Such carboxyl modification of a rubber typically involves the addition of about 0.75 percent to 15 percent by weight of an unsaturated carboxylic acid of the acrylic acid type to the monomer charge composition of the carboxylic rubber being synthesized.
These carboxylated elastomers can be vulcanized in a manner analogous to their uncarboxylated counterpart utilizing a sulfur curing agent. In addition to this, if a polyvalent radical and particularly divalent metals are available in the vulcanization recipe, the carboxyl groups in the polymer chain can take part in this cross-linking reaction. This cross-linking reaction is fast in the presence of divalent metals and scorch problems are often encountered. Even at room temperature, carboxylated rubbers will often cure in 48 hours or less in the presence of zinc oxide when uninhibited. Since scorch (the premature cross-linking of an elastomer) can render a rubber completely unworkable, it is necessary to control this cross-linking reaction between carboxyl groups on the polymer chain. This invention discloses the use of adipic acid as an agent to greatly improve the scorch resistance of carboxylated rubbers.